Carrageenan is a complex mixture of sulfated polysaccharides comprising linear polymers of 1→3 linked [α]-D-galactose units and 1→4 linked [β]-D-galactose units. Carrageenan is produced by red seaweeds where it functions as the principle structural polysaccharide. It is located within the cell wall and intracellular matrix of the plant tissue. The carrageenan content of commercially harvested seaweeds is generally between 30% and 80% based on the seaweed dry weight. Carrageenan finds wide applicability as a food ingredient and is functional in foods such as dairy products, water dessert gels, meat products, confections, beverages, dressings and other such products. Carrageenan is also useful in products such as cosmetics, toothpaste and other personal care products, in soft gel capsules, and in other industrial, medical, pharmaceutical, and agricultural applications. The molecular weight of carrageenan products is typically from about 100,000 to about 1,000,000 Daltons. Carrageenans have the ability to form an almost infinite variety of gels at room temperature with a variety of gelling and melting points. Carrageenan solutions can thicken, suspend, and stabilize particulates, colloidal dispersions and water/oil emulsions. The solutions shear thin, which allows them to be pumped easily. Also, the sheared solutions rapidly rebuild viscosity and suspending power upon standing. Depending upon the application, carrageenans present in parts per million up to a few percent by weight provide gelling, thickening, suspending, binding and/or generates a desired product feel or texture.
Carrageenan is generally soluble in warm water, in which it forms a viscous solution. It is insoluble in most organic solvents and typically forms complexes with proteins. The major types of carrageenan are designated as Kappa, Iota, Lambda, Nu and Mu. These are differentiated based on the nature of the repeating galactose units contained in the carrageenan. The polymer chain can be cleaved by hydrolytic depolymerization upon treatment with an acid or by oxidative depolymerization upon treatment with hydrogen peroxide.
In a typical process for producing refined carrageenan, crude seaweed is first washed with cold water or seawater to remove sand and other particles that may be present after seaweed has been harvested. Carrageenan typically does not swell during the cold wash, primarily because carrageenan in seaweed is associated with structural components of the seaweed, generally cellulose. Depending upon the seaweed species, following the cold wash, a hot water extraction process is typically performed in which the extracted carrageenan is treated with an aqueous base at high temperature. Generally, the base used is an alkali or alkaline earth metal hydroxide such as, for example, sodium hydroxide, calcium hydroxide, or potassium hydroxide. This high temperature aqueous base modification step leads to the formation of 3,6-anhydro linkages in the galactose units of the carrageenan. After this modification, the hot extract is filtered to remove insoluble materials such as cellulose, hemicelluloses and other particulates and acid is added to adjust the pH to 7.5 to 10.5. The filtrate can then be concentrated to about 4% carrageenan for further processing. Optional process steps after extraction include centrifugation and bleaching. Refined carrageenan is typically obtained by precipitation of the extract from the aqueous solution with potassium chloride or an alcohol such as isopropyl alcohol. The resulting carrageenan product is subsequently dried and ground.
Material throughput for production of refined carrageenan on a commercial scale is rate limited. After the extraction step, a hot aqueous stream can typically only contain low concentrations of carrageenan, typically up to about 4%. At higher concentrations of carrageenan, the aqueous stream becomes too viscous to process efficiently.
There has been an ongoing search for more cost-effective methods of preparing semi-refined carrageenan and other carrageenan products as lower cost replacements for conventional carrageenan. Semi-refined carrageenan (SRC) products are those in which few or none of the structural components of the seaweed, principally cellulose, have been removed. During production of SRC, a salt such as potassium chloride or sodium chloride is added during the base modification along with the base. The presence of sufficient amounts of salt prevents disintegration of the seaweed structure and inhibits extraction of carrageenan from the seaweed. An alcohol such as isopropyl alcohol can also be used alone or in combination with salt to inhibit extraction of the carrageenan. Following the base modification step, with the seaweed structure still intact, the processed seaweed is typically dried to afford SRC. When the seaweed is a member of the Euchema family, the SRC obtained is known as processed Euchema seaweed (PES).
U.S. Pat. No. 5,801,240 to Rideout et al. discloses a method for producing semi-refined kappa carrageenan. It describes the conventional PES production process with improvements including better monitoring of oxidation-reduction potential of the potassium hydroxide (KOH) solution used in the extraction method and optionally chopping the seaweed prior to KOH cooking. U.S. Pat. No. 5,777,102 to Larsen discloses a modified carrageenan made by partially hydrating seaweed in water/solvent/base mixture to get modification of the carrageenan following which the material is extruded. U.S. Pat. No. 5,502,179 to Larsen discloses a method in which seaweed is reacted under heating in a solvent/water/base mixture to get fully modified carrageenan followed by extrusion to obtain a product with a specific light transmission and Brabender profile. WO 03/059956 to Therkelsen discloses a heterogeneous carrageenan manufacturing process from mono-component seaweed with reduced use level of KOH. This is an improved cost effective method of making carrageenan extract or PES that uses high salt level in combination with NaOH to reduce the level of costly KOH. U.S. Pat. Nos. 6,479,649 B1, 7,018,635 B2, and 7,189,843 B2 to Tsai et al. disclose a method for production of carrageenan and carrageenan products with its main focus on extrusion of seaweed feedstock. U.S. Pat. No. 6,387,354 B1 to Bixler et al. discloses a binder for a toothpaste composition comprising semi-refined kappa and/or semi-refined iota carrageenans in combination with other binder components. Japanese Patent Publication No. JP8005921B2 discloses a modified iota carrageenan having a viscosity of 5 cP to 40 cP in a 1.5% aqueous solution at 75° C. In this disclosure, a part of iota carrageenan can be replaced by lambda carrageenan for a ratio of iota to lambda carrageenan that is 1 to 4 or higher. A mixture of these carrageenans in such a ratio when used as a binder exhibits an effect of stabilizing the quality of toothpaste nearly the same as obtained when iota carrageenan is used alone. Japanese Patent Publication No JP2752610B2 discloses a modified iota carrageenan with a viscosity of 5 cP to 40 cP at 75° C. obtained from Euchema spinosum by hydrolyzing its extract with alkali hydrolysis, acid hydrolysis, hydrolysis using an oxidant, or hydrolysis with an enzyme or other microorganisms. When used as a stabilizer in toothpaste formulations, this modified iota carrageenan is preferably compounded with materials like calcium phosphate, glycerol etc. Japanese Patent Publication No. JP2752611B2 discloses toothpaste comprising lambda carrageenan and modified carrageenan the aqueous 1.5% solution of which has a viscosity of 5 cP to 40 cP at 75° C.
Refined carrageenans have some advantages to SRC obtained from the same seaweeds. Refined carrageenans typically hydrate, i.e. begin to swell and become soluble, at lower temperatures than SRC. Another advantage of refined carrageenans is that because SRC contains cellulosics and other materials which are absent in refined carrageenans, gels formed from SRC have a lower clarity compared to gels prepared from refined carrageenan products. Certain semi-refined carrageenans have also experienced stability issues in toothpastes. There remains a continued need for development of a toothpaste having acceptable and/or improved shelf life and stability using semi-refined carrageenan as a replacement for some or all of the refined carrageenan.